ABSTRACT
Early co-transcriptional events during eukaryotic ribosome assembly result in the formation of precursors of the small (40S) and large (60S) ribosomal subunits. A multitude of transient assembly factors regulate and chaperone the systematic folding of pre-ribosomal RNA subdomains. However, owing to a lack of structural information, the role of these factors during early nucleolar 60S assembly is not fully understood. Here we report cryo-electron microscopy (cryo-EM) reconstructions of the nucleolar pre-60S ribosomal subunit in different conformational states at resolutions of up to 3.4 Å. These reconstructions reveal how steric hindrance and molecular mimicry are used to prevent both premature folding states and binding of later factors. This is accomplished by the concerted activity of 21 ribosome assembly factors that stabilize and remodel pre-ribosomal RNA and ribosomal proteins. Among these factors, three Brix-domain proteins and their binding partners form a ring-like structure at ribosomal RNA (rRNA) domain boundaries to support the architecture of the maturing particle. The existence of mutually exclusive conformations of these pre-60S particles suggests that the formation of the polypeptide exit tunnel is achieved through different folding pathways during subsequent stages of ribosome assembly. These structures rationalize previous genetic and biochemical data and highlight the mechanisms that drive eukaryotic ribosome assembly in a unidirectional manner.
Subject(s)
Cell Nucleolus/chemistry , Cryoelectron Microscopy , Ribosome Subunits, Large, Eukaryotic/metabolism , Ribosome Subunits, Large, Eukaryotic/ultrastructure , Saccharomyces cerevisiae , Cross-Linking Reagents/chemistry , Models, Molecular , Molecular Mimicry , Protein Domains , Protein Stability , RNA Folding , RNA, Ribosomal/chemistry , RNA, Ribosomal/genetics , RNA, Ribosomal/metabolism , RNA, Ribosomal/ultrastructure , Reproducibility of Results , Ribosomal Proteins/chemistry , Ribosomal Proteins/metabolism , Ribosomal Proteins/ultrastructure , Ribosome Subunits, Large, Eukaryotic/chemistry , Saccharomyces cerevisiae/chemistry , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/ultrastructure , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae Proteins/ultrastructureABSTRACT
We describe an enhancement of traditional genomics-based approaches to improve the success of structure determination of membrane proteins. Following a broad screen of sequence space to identify initial expression-positive targets, we employ a second step to select orthologs with closely related sequences to these hits. We demonstrate that a greater percentage of these latter targets express well and are stable in detergent, increasing the likelihood of identifying candidates that will ultimately yield structural information.